Date: 2026-09-18 hits: 101
I. Fatal Chemical Reactions in the Electrolyte
The danger of excessive moisture stems from its chemical reaction with the electrolyte's core component, lithium hexafluorophosphate (LiPF6).
Moisture reacts with LiPF6 to produce hydrofluoric acid (HF) and phosphorus oxyfluoride (POF3). The reaction equation is: H2O + LiPF6 → POF3 + LiF + 2HF.
HF is a highly corrosive acid that attacks almost all internal battery components. Simultaneously, the reaction consumes the active lithium salt in the electrolyte and generates non-conductive by-products such as LiF inside the battery; LiF deposits on the electrode surface, increasing interfacial impedance. This reaction also produces gases like CO and CO2, which are the direct cause of battery swelling.
Notably, the side reaction between moisture and the electrolyte accelerates at high temperatures, explaining why batteries fail more rapidly in high-temperature environments.
II. Specific Damage to Various Lithium Battery Components
1. Cathode: Structural Corrosion and Loss of Active Material
Material Corrosion: HF directly corrodes cathode materials, causing the dissolution of transition metal ions (e.g., Ni, Co, Mn). High-nickel ternary materials, in particular, are highly susceptible to moisture absorption due to their high surface alkalinity, reacting to form LiOH and Li2CO3. These by-products contribute no capacity and obstruct lithium-ion pathways.
Increased Internal Resistance: As moisture content rises, POF3 and LiF precipitate on the surface of the SEI (Solid Electrolyte Interphase) film. This compromises the film's density and uniformity, leading to a gradual increase in internal resistance and a continuous decline in discharge capacity.
Performance Degradation: The most immediate consequences are capacity loss and shortened cycle life. Excessive moisture damages the SEI film on the anode surface, reducing electrode interface stability and causing a continuous rise in internal resistance. Increased internal resistance not only directly causes discharge capacity decay and reduces usable runtime per charge but also accelerates performance degradation during cycling, ultimately significantly shortening the battery's effective cycle life. 2. Anode: SEI film damage and interface failure
The dual nature of moisture regarding the anode SEI film: An appropriate amount of moisture facilitates the formation of a uniform, dense SEI film composed primarily of Li2CO3. However, excessive moisture compromises the stability and uniformity of the SEI film.
2. Anode: SEI film damage and interface failure
The dual nature of moisture regarding the anode SEI film: An appropriate amount of moisture facilitates the formation of a uniform, dense SEI film composed primarily of Li2CO3. However, excessive moisture compromises the stability and uniformity of the SEI film.
(1) Gas generation
Moisture reacts with the electrolyte (e.g., LiPF6) to produce HF and CO2; gas accumulation causes cell swelling, a risk that intensifies at high temperatures. HF corrodes and damages the anode SEI film, exposing the anode surface to continuous reductive side reactions with the electrolyte. This generates large quantities of gases such as H2, CO, CO2, and C2H4. Simultaneously, the repeated rupture and repair of the SEI film continuously consume active lithium and electrolyte, creating a vicious cycle that ultimately leads to cell swelling, a sharp rise in internal resistance, a precipitous drop in capacity, and premature termination of cycle life.
(2) SEI film damage and lithium dendrite growth
HF reacts with and disintegrates the primary components of the SEI film (such as ROCO2Li and Li2CO3). Once the SEI film is damaged, the fresh anode surface is re-exposed to the electrolyte, leading to the continuous consumption of active lithium ions and electrolyte to repair the film. This repair process causes a sustained increase in internal resistance, greater irreversible capacity loss, and exacerbated gas generation. The damaged anode surface may even develop a black, water-induced film layer that severely hinders lithium-ion intercalation.
3. Separator: Physical damage and potential risks
The separator itself is not directly sensitive to water, but moisture-induced chain reactions can damage it indirectly.
(1) Chemical corrosion: The strong acidity of HF corrodes the separator, damaging its microstructure and increasing the risk of leakage or short circuits.
(2) Physical clogging: Solid by-products generated by reactions, such as LiF, may deposit within the separator's pores, blocking lithium-ion transport channels and increasing internal resistance.
(3) Reduced thermal stability: Corroded or clogged separators exhibit diminished thermal stability and mechanical strength; under abnormal conditions, they are more prone to shrinkage or rupture, potentially triggering internal short circuits.
4. Electrolyte: Loss of Function and Quality Degradation
The electrolyte is the primary site of attack by moisture, and it undergoes degradation as a result. Loss of active components: Moisture reacts directly with LiPF6, consuming the key component that supplies lithium ions. Simultaneously, the reaction generates large amounts of HF, causing the electrolyte's acidity to rise sharply and increasing its corrosiveness. Overall performance deterioration: The degraded electrolyte exhibits reduced ionic conductivity, leading to a comprehensive decline in the battery's rate capability, low-temperature performance, and safety.
(1) Chemical Reactions and Corrosion
Excessive moisture in the electrolyte reacts with lithium salts (such as LiPF6) to produce hydrofluoric acid (HF): H2O + LiPF6 → POF3 + LiF + 2HF. HF is a highly corrosive acid that attacks the battery's internal cathode and anode materials, current collectors (e.g., aluminum and copper foils), and separators; this damages the battery structure and increases the risk of leakage and short circuits.
(2) SEI Film Damage and Increased Internal Resistance
Excess moisture damages the Solid Electrolyte Interphase (SEI) film on the anode surface, leading to increased interfacial impedance. This hinders lithium-ion intercalation, raises the battery's internal resistance, reduces discharge capacity, and shortens cycle life.
(3) Gas Generation and Battery Swelling
Gas generation and swelling caused by excessive internal moisture: Moisture reacts with the electrolyte to produce gas, raising the internal pressure of the battery. This can lead to swelling, casing deformation, or even rupture. The common phenomenon of mobile phone battery swelling often stems from this.
(4) Lithium Plating
HF attacks the SEI film; fresh graphite exposed after the film ruptures immediately reacts with the electrolyte in an attempt to reform the SEI layer. However, each repair consumes active lithium ions and solvent from the electrolyte, causing a sharp increase in interfacial impedance and sluggish reaction kinetics. Ohmic polarization and electrochemical polarization increase significantly. The actual potential of the anode drops rapidly, eventually causing lithium ions to gain electrons at the anode surface and form metallic lithium.
III. Deterioration of Overall Electrochemical Performance
The damage to the four key components described above ultimately manifests in the battery's overall performance.
Capacity Drop: The consumption of active lithium and corrosion of the cathode directly result in significant capacity degradation.
Increased Internal Resistance: Damage to the SEI layer, separator clogging, and electrolyte degradation collectively cause a sharp rise in internal resistance.
Cycle Life Degradation: Increased internal resistance and the loss of active materials drastically shorten the battery's cycle life.
Heightened Safety Risks: Internal gas generation leads to battery swelling and deformation; HF corrosion can trigger internal short circuits; and all these factors pave the way for thermal runaway within the cell.
IV. Summary
In summary, excessive moisture in lithium batteries is a critical issue. It triggers a chain reaction by compromising the electrolyte and generating corrosive HF, which subsequently corrodes the cathode, breaks down the SEI layer, contaminates the separator, and degrades the electrolyte. Ultimately, this leads to a sharp drop in capacity, increased internal resistance, and reduced cycle life, while creating serious safety hazards such as swelling, leakage, and even fire or explosion.
Therefore, strict control of moisture throughout the entire process—from material storage and electrode fabrication to electrolyte filling—is the cornerstone of ensuring lithium-ion battery safety.
